The invention relates to a process for the manufacture of filaments wholly or substantially consisting of aromatic para-positioned polyamides.
In a typical process, fibers are made from an aromatic polyamide such as poly-paraphenylene terephthalamide, polyparabenzamide, or poly-4,4'-diaminobenzanilide terephthalamide (4,4'-DABT), by spinning a spinning mass consisting of a mixture of concentrated sulphuric acid and, calculated on the weight of the mixture, 16 to 30% of the polymer with an inherent viscosity of 3,5 to 7 or higher, the spinning mass being extruded downwardly into a coagulation bath from a spinning unit provided with spinning orifices, of which spinning unit the outflow side is positioned in a gaseous, inert medium, preferably air, and at a short vertical distance, of, say, 2,5 to 25 mm, from the liquid surface of the coagulation bath, and the filaments are withdrawn from the coagulation bath followed by subjecting them to a few aftertreatments, such as washing, drying and/or winding. Poly-para phenylene terephthalamide will be referred to hereinafter as PPDT.
A process for the type indicated above is disclosed, among other places, in U.S. Pat. No. 4,078,034 and U.S. Pat. No. 4,340,559. Both said well-known process and the process according to the invention relate in part to the manufacture of PPDT filament yarns and/or fibres with a relatively high tensile strength and a high modulus of elasticity. After having been commercially available for several years, these yarns increasingly find application in various high-grade products on which high demands are made as far as physical properties and other qualities are concerned.
Examples of high-grade products include reinforcing cords for vehicle tyres, conveyor belts, cables, ropes, etc. On economic grounds it is desirable that the highest possible production capacity should be attained. To increase the speed of production of aramid yarn it has in the first place been proposed that the winding speed be increased, i.e. the speed at which the completed yarns are wound into the form of a package. However, as mentioned in U.S. Pat. No. 4,340,559, an increase in winding speed is attended with deterioration of the physical properties and the quality of the yarn, particularly when yarns composed of a large number of filaments are to be produced. It has now been found that these drawbacks to increasing the speed are due to the PPDT process being fairly critical; in particular as regards the relatively short height (typically 2,5 to 25 mm) of the air gap between the underside of the spinneret and the spinning bath level and the use of a relatively shallow coagulation bath, which generally has a depth of 15 to 40 mm. Consequently, at higher spinning speeds and, corresponding, higher winding speeds, the residence time of the freshly spun filaments in the air gap and in the bath will become very short. Another cause of said problems is that in the well-known spinning processes for PPDT a fairly deep funnel-shaped depression is formed at the point where the bundle of, say, 250 to 1000 filaments enters the bath, as can been in for instance FIG. I and IV of U.S. Pat. No. 4,340,559. The centre line of said depression coincides with the line connecting the centre of the spinning unit and the centre line of the bath outlet for the filaments. The higher the speed of the filament bundle, the greater will be the depth and the width of said depression. The formation of such a depression will give rise to a relatively great average increase in air gap and an average decrease in bath depth at the filament bundle. Further, there will be differences between the distances covered through the air zone and through the bath between the filaments on the outside and in the inside of the bundle.
In air gap spinning PPDT there is the problem that a considerable amount of the bath liquid usually consisting mainly of water and sulphuric acid will escape from the bath through the outlet opening for the freshly spun filaments positioned below the surface of the bath, which is particularly dependent on the height of the liquid column above the outlet opening. But the discharge of bath liquid through the outlet opening is to a very large extent effected in that the liquid is drained off along with and between the advancing filaments of a bundle via the outlet opening. This means that the amount of liquid entrained out of the bath along with the filament bundle will strongly increase with increasing winding speed from more than 350 m/min and up to 3000 m/min. Further, with a view to increasing the production capacity the number of filaments extruded into the bath at each spinning position will be increased as much as possible from more than 1000 to between 3000 and 10,000 filaments, which is another cause of a greatly increased amount of liquid being discharged from the bath through the outlet opening. The escape of large amounts of bath liquid through the outlet opening for the filaments will first of all make it necessary for an at least equally large amount of liquid to be re-fed to the bath or to be circulated. Moreover, said large stream of liquid via the outlet opening will result in the occurrence in the bath of undesirably high flow rates or turbulencies.
To diminish the amount of liquid discharged through the outlet opening it is possible in principle to reduce the area of the outlet opening. This has the disadvantage, however, that stringing up the filament bundle before re-starting the production process, for example after filament breakage, becomes a particularly difficult and time consuming operation, resulting in loss of production. This solution to the problem is all the more objectionable in that in the case of air gap spinning PPDT filaments the stringing up operation is in principle not simple at all because of the very limited space available below the spinneret and the emerging very agressive spinning solutions. With a known process it has been proposed before that stringing up be effected using an injector. Such a system, however, is complicated and not at all trouble-proof.
As far as the background of the state of the art is concerned reference is made to FR No. 1 071 888, GB No. 922 485, FR No. 703 114 and U.S. Pat. No. 2,228,155. They disclose the extrusion from the spinning unit of two or more separate groups of filaments of different materials for other spinning processes. Unlike the process of the present invention these well-known spinning processes do not relate to air-gap spinning, in which the extruded filaments first pass through an air zone and subsequently through a spinning bath. In other words, the spinning processes according to the above patent specifications do not relate to the air-gap spinning process, which is fairly critical for the spinning of poly-paraphenylene terephthalamide, particularly as regards the relatively small width of the air gap between the underside of the spinneret and the surface of the spinning bath of a relatively shallow coagulation bath. The spinning of two or more separate filament groups in the spinning processes according to said four disclosures is therefore not used for solving the PPDT air gap spinning problem of the undesirable formation of funnel-shaped depressions in the coagulation bath.
Reference is also made to Japanese Patent Specification publication No. 7 019 413, which describes a process for spinning fibres from polyacrylonitrile. In that case the spinneret is placed above the spinning bath at a distance from it of 1-10 mm and the object is to make filaments having an irregularly shaped cross-section, to which end the spinneret is provided with a large number, say 26, of groups of spinning orifices, each group counting for instance two or three orifices. The spinning orifices in each group are spaced at intervals of 0.1-0.7 mm, the distance between the groups being at least 1 mm. The irregular cross-sectional shape of the filaments is to be attributed to the fact that the two or three freshly extruded filaments in each group adhere to one another. This sticking together of filaments in the same group does not occur in the PPDT spinning process of the present invention and would lead to a qualitatively unacceptable product. The spinning process of said Japanese patent specification therefore greatly differs from the spinning process of the present invention. The envisaged effect of said Japanese patent specification, viz. sticking together of filaments, might prejudice a skilled man against applying the well-known process or at least a variant thereof, in the air gap spinning of PPDT.
One method of increasing the winding speed of the fibers during production is to use in the spinning mass a polymer which was produced by a process employing hexamethylphosphoramide as a solvent. The spinning mass will then have traces of this solvent (or its decomposition products), which apparently has lubricating effect during spinning, thus permitting higher winding speeds. However, hexamethylphosphoramide is generally considered to be a carcinogen, and it would be highly advantageous to provide a process for obtaining high winding speeds without its use.